From: Denver Colorado
I don't think that's going to happen until over-land rail connects.
I also don't think you can move an FBD and start rail-repair work there unless overland rail is adjacent (Germany's not going to put trains on ships, move them to Riga, offload, and start doing rail supply from there).
Units may draw supply from a port, though, and it will display in their supply details but I'm not sure if it will list Riga as the supply source or a German port connected to the rail, like Konigsberg for example.
Summer 2017-Playing: D-Day at Omaha Beach, Advanced Squad Leader,
Reading: Kampfgruppe Walther & Panzerbrigade 107 (Magnificent). Lots of Osprey stuff.
Rulebooks: ASL (always ASL)