Yes to both.

1: A host if it successfully captures land can form into a realm (Though doing so may piss off people if the land did not originally belong to you) similar to the first crusade forming the kingdom of Jerusalem. ALSO, the reverse is possible, if a realm is taken over by the darkness (And is likely to happen), a realm player can continue playing as a host of their realms remaining soldiers and/or refugee's. Note that a host does not HAVE to form a realm if it retakes land and continue on crusading (Usually resulting in the land going back to its original owner) or if a host is defensive it simply continues on with its mission.

2: Hosts can recruit from realms through two means. 
Officially: Through "Patron Realms" (Realms that officially sponsor hosts), a host can build chapter houses/strongholds/forts/whatever to act as bases of operations. Similar to how the Knights Templar owned numerous facilities across Europe, a host can use these strongholds as bases of operations, mustering area's, recruitment, training and civilian headquarters for branches of the host that deal with things like logistics, finances and negotiations.
Unofficially: By hiring mercenary bands, hiring specialized mercenary companies, or if the host is particularly religiously motivated, convince members of their faith to join up. Unofficial recruitment also includes antagonistic acts against the nation they are passing through, like directly recruiting peasants or soldiers without permission from the lords of the land. This kind of recruitment behavior can escalate into infighting (Note: Fighting between realms and hosts is very possible and may be a chief factor in the forces of darkness expanding)