Oh!  In that case, the formula for H should change.
Effectively, you are making an additional contribution
once a year in the amount of the tax refund, which then
compounds monthly.  At this point, it might be easier
to make the simplifying assumption that the investment
period, B1, is in years (again), not months.  Sorry
for the flip-flop.

Thus, if A1 is the nominal annual rate of return, B1
is the number of years of the investment, C1 is the
monthly contribution, and D1 is the tax rate, then:

G1:  =FV(A1/12, 12*B1, -C1,, 1)

H1:  =G1 + FV(EFFECT(A1,12), B1, -12*C1*D1)

EFFECT() gives the effective rate at which an annual
contribution compounds monthly.  For example, 7.50%
if the annual rate is 7.25%.

If EFFECT() does not work for you, you need to load
the Analysis ToolPak add-in.  See footnote [1] below
for alternatives.

Granted.  But this brings to mind a curiosity that I
wanted to ask about RRSPs.  I think you answered it
implicitly; but I want to be sure.  Is the profit earned
within an RRSP "tax deferred" (taxed later) or truly
"tax free" (never taxed!)?

I presume it is tax deferred, like the US IRAs; that is,
I assume you will be taxed on the profit when you withdraw
it.  But the "best" web site I find that explains RRSPs
(to a minimal degree) uses the term "tax free" -- which
would mean that the profit is never taxed(!).  This is
a common terminology mistake (or misleading statement)
that US brokers often make, too.

I do not know of any Excel function that would facilitate
this.  It might exist -- perhaps some convoluted use of
SUMPRODUCT, a favorite of many respondents in these
newsgroups.  I simply am not aware of it.

I would create a row for each investment year, with
columns similar (but not exactly the same) as above --
although you could simplify it by putting some "constant"
values in other cells.  Here is how I set things up:

A1:A10 = annual return rate (7.25%)

B1:B10 = remaining number of years for investment; for
             example: B1=10, B2=9, ..., B10=1

C1:C10 = monthly contribution; for example: C1=1000,
              C2 =ROUNDDOWN(C1*(1+1%), 0), copy C2 to C3:C10

D1:D10 = tax (credit) rate (36.9%)

E1:E10 = year-end value of the year's contributions,
             compounded monthly; for example:
             E1 =FV(A1/12, 12, -C1,, 1), copy E1 to E2:E10

F1:F10 = the year's tax credit/refund due to RRSP
             contributions; for example:
             F1 =12*C1*D1, copy F1 to F2:F10

G1:G10 = future value (FV) of E1:E10 respectively,
              compounded monthly for the remaining years;
              for example:
              G1 =FV(EFFECT(A1,12), B1-1,, -E1),
              copy G1 to G2:G10

H1:H10 = G1:G10 plus FV of reinvested F1:F10
              respectively, compounded monthly for the
              remaining years; for example:
              H1 =G1 + FV(EFFECT(A1,12), B1-1,, -F1),
              copy H1 to H2:H10

In row 11, I compute the following sums:

C11 = total monthly contributions; for example:
         =12*SUM(C1:C10)

G11 = total FV of investment; for example:
         =SUM(G1:G10)

H11 = total FV of investment and reinvested tax
         credit; for example:  =SUM(H1:H10)

Of course, all that makes some simplifying assumptions
-- probably good enough for a "first approximation".

You're welcome.  Now, let's talk about your transgender
issues, Ryan/Gretchen :-).

-----
[1] Footnote regarding EFFECT().

Alternatively, replace EFFECT() above with
FV(A1/12,12,,-1)-1.  I prefer the latter because it
saves me the trouble of getting Help on the function
when I forgot its parameters, since Excel provides
typing hints for intrinsic functions, but not for
ATP functions :-( , and because it takes less effort
than explaining to people why EFFECT() sometimes fails :-).
For similar reasons, I prefer to use FV() instead of
the geometric growth expression (1 + 7.25%)^12 - 1.

If that paragraph leaves you gasping for air, just
ignore it ;-).