Q:

How can I implement multiclassing?

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.

A:

Rogue Trader's existing solution to the problem of not having a player playing each of the available classes is to assume that the "missing party roles" are made up by nameless, faceless NPCs.
After all, the player characters in Rogue Trader are effectively the privy council of a ship with a population of thousands. These multitudes include multiple tech-priests, shiphands experienced enough to be called void masters, and at least one navigator, or the ship wouldn't be able to keep the lights on, let alone move.
These crew members might not appear "on camera" very often, and aren't part of the trusted coterie of the Lord-Captain that decides what the ship does, but they're assumed to be on board unless the GM declares otherwise. As such, it's not necessary to require player characters to fill those roles. (Also, players taking particular roles indicates that they're interested in doing what those roles do; Requiring them to do other things might not be fun for them.)

