I have switched between sql server and advantage server while developing applications and I have mainly used the import data function within the sql server management studio to do this. Basically create a database and import data using the oledb driver for advantage - available from the advantage database website. Try this link for more information . Going back again is a bit tricker.... but that is another story.
If it is just a one off transfer that should do the trick you will have a fair degree of control over transfer. Be warned that the information about PK and data relationships may not survive the transfer as at least with adt/adi files advantage database server does not keep a record of the relationships between tables. If they want to keep the advantage database running and keeping a copy of the data in both databases i have not tried to do. The suggestion from OMG Ponies may do the trick in that case.