Risk in creating a string domain
Yes you can create create a domain/type as a work around  but I caution against this 

All future upgrades have to maintain this new datatype that added no new feature only overhead.

Converting Access/MSSql SQL queries and VB scripts to PLpg/SQL is a complete rewrite there is no way around that..

There are enough subtle/major difference between MS implementation of SQL and every other implementation to make it not work with any other database at all.  

Only the most simplest queries will can be reused

Select * from sometable


but MS typically generates

Select * from [schema].[table]