unfortunately the active document cannot really be set as it is the current drawing in cad. If you need to open/use another file, you can do it tho.
you do it in paralell to your current drawing.
dim dbOutside as ACDB.Database = new ACDB.Database(false, false)
dbOutisde.ReadDwgFile("", System.IO.Filshare.ReadWrite, True, "")
'this line is real easy to follow, put the filepath param to where ever the file is. the last pararm is a password var, but such things are rarely used.
'now use that db just like standard db, liek many sample here use the working database of the current drawing.