Hi Jason,
unfortunatly a select list cannot access a web service. You need to have some workaround to use the values from Dynamics NAV.
A select list can either access a text file or a database table to show the entries, so somehow you need to access the NAV database directly or copy the data from NAV to another database which can be accessed from the DocuWare system.
I hope this helps a little bit.
Regards
Tobias