What do you hope to accompish with this?  If you think you'll be able to
specify which account will be used when sending to those contacts, you are
mistaken.  If you're just trying to group contacts to segment them, use
categories.

That said, you can certainly add an account value to a selection of
contacts.  DIsplay your contacts in a table list, like By Category.  Then
use the Field Chooser to add the Account field from the "All Contacts
fields" selection.  Then enter some value in that field for one contact.
Right-click the Account column header and select "Group by this field".  Now
you'll have two groups: the group containing the account value you just
entered, and all other contacts.  Select as many contacts as you wish with
CTRL-Click or Shift-Click and then drag the selection to the group you
created.  All those contacts will have that account value added to therm.