It is mentioned in a hadith that if a believer goes through suffering, grief, or pain, it expiates his/her sins.

Based on this, if a person suffers as a consequence of some sin or wrong/illegal decision and repents, but still continues to suffer the consequence of it, will that suffering and pain be a source of pardon and forgiveness for him?

There are many ways Allah may choose to forgive someone and one of them is any suffering, grief or pain a believer goes through. Through this Allah forgives His servants, if it is met by the servant with patience.

'A'isha radiAllahu anha reported:

I heard Allah's Messenger (ﷺ) as saying: “There is nothing (in the form of trouble) that comes to a believer even if it is the pricking of a thorn that there is good decreed for him by Allah or his sins are obliterated.” [1]

Reference:

[1] Sahih Muslim 2572