they. Not the "law" and "commandments" of Proverbs 3:1, for they are feminine, but the "days" of Proverbs 3:2 (which are masculine, agreeing with the verb "add", which is masculine also). See App-74.
add to thee. This, in the Hiphil. make increase for thee, or cause thee to increase or grow [in wisdom]: i.e. as the days and years lengthen and increase they will add to thy wisdom if thou forget not, &c. So in other passages where wisdom is supposed to promise long life, which it does not. See notes on Proverbs 3:16; Proverbs 4:10; Proverbs 9:11; Proverbs 10:27.