Can you say 「普段に○○」 or does it have to be「 普段は○○」?
Both are possible but in (subtly) different contexts.
Roughly 普段に means for non-special occasions, daily uses and its implication is that something different is used for special occasions. 普段は means usually, with implication being that the speaker is doing otherwise for some reason.
普段に(1) means I wear this sweater for non-special occasions. The focus of the sentence is simply on this sweater, and it is saying that the sweater is nothing special and the speaker wears it for daily use. Practically, you see it in combinations like 普段に使う, 普段に着る etc. A relevant related phrase in this sense is 普段使い.
普段は(2) means I usually wear this sweater, and the implication is that the speaker will wear something different for some reason (by the function of は, you may already know this or can search on this site).
An additional set of examples to clarify the difference:
3 is rather unlikely unless the speaker is someone who has a party almost everyday. 4 is fine (assuming the speaker is a party goer) and it means I wear this dress normally (on a occasion like this or for most parties).
Note that all of the above sentences (esp. 1) may sound a little strange by themselves. They would appear more like このセーターを普段に着るのは躊躇する I hesitate to wear this sweater daily (because I cherish it so much or it is so expensive).