So far I had never found "somehow" used with "in". Is still this the normal adverb? Could I use "somehow" alone?
Ex: I'll fit you in somehow.
The preposition "in" goes along with fit and not with somehow. It is separated from the verb fit on the account of English syntax And - of course - "in somehow" by itself, would be ungrammatical.
There's an implied noun after the preposition in, most likely "my schedule" or "the list of people involved" or something similar, that's elided. So, "I'll fit you in" means "I'll fit you in my schedule". (It could well be "I'll fit you in my really skinny jeans" or something completely different, but barring context telling us that, the schedule sense is the most likely).
In "I'll fit you in my schedule, somehow" the somehow would be modifying the verb fit, suggesting that while fitting you into my schedule will not be easy, I will still find a way to manage it.
"I'll fit you in somehow" hence means the same. "I'll fit you in, somehow" is probably clearer.