default value being a concatenation of other columns

Hello all,

Does anybody know if it is possible to set the default value of a
field to equal the concatenation of 2 other fields?
I.E.
Field 1 = 123
Field 2 = 345
Field 3 = (Field1 + Field2) = 123345

This is easy enough to do using a stored procedure, but I'm trying to
use the import tool and so need to do the concatenation by default.

Many thanks,
Drew.