Deaf myself: usually we start with fingerspelling. If we discover it being cumbersome we invent something. Borrowing is different but not impossible.

Not to mention the new signs can involve the hand shapes from, say, the first letter of the borrowed English word. Say the word starts with a D, and everybody recognises the fingerspelt word just from the D alone (in context), then the new sign might use the hand shape for D but give it a specific action.