Can we use past continuous tense to refer a past habit with adverbs of frequency words? For example:
He was going for a morning walk every day.
This is indeed correct. Past continuous can be used for referring to actions that habitually occurred in the past but now no longer occur. For example, "at the time, his marriage was a recent event, and he was having sex with his new bride two or three times every day". Or: "Before the main battles got started, the General was noted to be rigorously passing muster on his troops morning after morning". Or even: "Before the operation, he was not noted to be following his doctor's recommendations on diet assiduously or frequently." This is a completely acceptable usage.