I think you are exactly as presumptive, assuming that Klingons would not add Klingon suffixes to foreign-language words as you are saying that Klingon speakers have been by doing so.

The simple truth is: In English, we don’t consistently use English suffixes on foreign words, but Klingon isn’t English. Until we see a canon example of it being done one way or another, we don’t know how it's done in Klingon.

This is an issue regarding one's particular style guide, not a grammatical or sociolinguistic issue. Different style guides may advocate different rules. The Kronos Manual of Style may advocate for keeping native and foreign elements out of single words, while the Praxis Memorial Guide to Style may advise always using native morphemes on foreign root words. Neither is "correct"; style guides are prescriptive.

What Qa'yIn has stumbled across is not a "rule," but it's definitely what I consider good style.

-- 
SuStel
http://trimboli.name