Inserting a few explanatory words is fine, but you should use square brackets instead of parentheses. Just take care not to alter the original text's meaning.
Writing Commons offers the following:
Quotation with brackets used correctly around an explanatory insert:
“[D]riving is not as automatic as one might think; in fact, it imposes a heavy procedural workload [visual and motor demands] on cognition that . . . leaves little processing capacity available for other tasks” (Salvucci and Taatgen 107).
Note: Brackets are placed around the inserted words in this example to
provide further explanation of the “procedural workload” discussed in
the original text.
Source: https://writingcommons.org/article/inserting-or-altering-words-in-a-direct-quotation/