Lookup on IMongoQueryable - getting an IAggregateFluent from IMongoQueryable

When accessing my data, I’m working with IMongoQueryable (the permission layer returns an 'IMongoQueryable? that restricts the executing user to see what they’re allowed to see). While I’ve managed to get my lookups going on an IMongoCollection (Generic Join for one-to-one and one-to-many relationships - #2 by Stephan_Steiner) I’m now looking to do the same on an IMongoQueryable. Is there a way to do so?