I suppose the equivalent is (what I now know is called) finger spelling: an unnatural 'sounding' word that breaks the flow, but it's [spelling is] taken from a language where it already exists.

See how well spelling out words rather than pronouncing them works for you in conversation. This really is a unique problem to adopting spoken words into a non-vocal language.

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.

Deaf myself: Often you can incorporate a fingerspelled letter into a sign, so some borrowing still happens. Where I live mouth pictures are important, so often you sign the general concept and mouth a part of the complicated word.