I don't think you will be able to do this using fields.

However, if your printer is an HP compatible laserjet you may be able to
send PRINT commands via a conditional field to switch the trays. I am not,
however, confident that it will work as part of the merge and I don't have
the printer available here to test it.

Also the actual commands vary between printers which is particularly
unhelpful, but if you can locate the bin escape sequences in your printer
handbook, something along the lines of the following - placed at the top of
the page - is worth a try

{IF {Mergefield Fieldname} = result1 "{PRINT 27"&l1H "}" "{IF {Mergefield
Fieldname} = result2 "{PRINT 27"&l2H "}" "{PRINT 27"&l3H "}"}

See also fellow MVP Jonathan West's excellent article on controlling
printers from vba -
http://pubs.logicalexpressions.com/Pub0009/LPMArticle.asp?ID=101

You could also split the merge document into separate letters if that would
be useful - http://www.gmayor.com/individual_merge_letters.htm