if key exists: update, otherwise: assoc

Use fnil to handle the nil value of v when k doesn't exist in the map:

(update m k (fnil conj []) v)

While the fnil answer is more spectacular, I find the following easier to read, especially if unfamiliar with fnil:

(assoc m k (conj (m k []) v))

where (m k []) returns the value of k in m or defaults to [] if k does not exist in m.

If k is a symbol, (k m []) would also work.