What kind of database are you using?
Rather than asking if you can use a certain method to do something, consider describing requirements and allow others to suggest a solution rather than pigeonholing them into your method.
The way I would address this would be to create a SQL Expression to return the first and last name concatenated, and then use the Set Default Values->Browse Table and Browse Field to point at the SQL Expression to pre-populate the pick list. Note that Crystal parameters are not dynamic, so you won't have the picklist automatically updated.
Since I don't know the database used, I can't supply the actual syntax or if your database supports this. Consider posting akin to a mini spec, try supplying:
Crystal version
Database/connectivity used
Example data
Expected output
You might try:
RTRIM("EMPLOYEE"."LAST_NAME") + ' ' +"EMPLOYEE"."FIRST_NAME"
Substituting your fields.
-k