My party is composed of 4 players, a Rogue Trader, an Arch Militant, an Astropath and an Explorator.
While to me this number is perfect, as it's not too much and not too few, I can see there are some classes that will be missing and are quite important (Void Master and Navigator, mostly).
I know I can simply use NPCs to fill the gaps, but I thought it might be interesting to apply some sort of multiclassing system so they could experience all the facets of the game themselves. For example, we lack a stealthy guy, so somehow, could we merge the Rogue Trader's skill/talent advances with the Seneschal? Or maybe the Astropath's with the Missionary?
If so, how can I do this? Spending a sum of XP to "acquire" a second class, a special feat that needs to made, or reaching a certain level?
PS: I don't quite like the idea of a player having more than one PC in a same game, so I have rejected that idea already.