The adverb form of narrow is narrowly, and like adverbs in general it doesn't have comparative forms - so you have to say more narrowly. However, that would give you:
Your hands are too far apart. Grab the bar more narrowly and chin up.
That would probably be understood, but I wouldn't consider it idiomatic. The natural way to say it is to use more words, for example:
Your hands are too far apart. Bring them closer together and chin up.
Now, you could use narrow as a verb, and people would understand the following:
Your hands are too far apart. Narrow them up and chin up.
That is better than using narrow as an adverb, or using more narrowly, but is not as good as bring them closer together. You could also use close them up in the same way as narrow them up (that's close in its usual verb sense, not pronounced the same as close meaning near), which would be slightly more natural than narrow them up.