What is incumbent upon one who had to make up days from Ramadan and did not do so due to successive Ramadans, and she does not remember whether she made them up or not, nor did she offer food in expiation for them? And is it permissible to observe voluntary fasting with the intention of making up for days she forgot?

It is obligatory to make up the days that are doubtful regarding their fasting, because one's obligation is not discharged except with certainty.

If the delay in making up the fast was due to an excuse or forgetfulness, then you only need to make up the fast. If it was without an excuse and you delayed it until the next Ramadan (definition), then you must make up the fast along with feeding a poor person for each day.

Charity given with the intention of expiation is sufficient if it is given to the poor and needy and it fulfills the value of feeding a poor person for each day. Some scholars are of the view that the monetary value suffices for the feeding. However, if the charity is given to those who are not poor, or if it is given to them but does not fulfill the required amount, then it is not sufficient and you must complete the deficiency.

It is not permissible to combine the intention of a voluntary fast with the intention of making up for Ramadan fasts, because each of them is an independent act of worship that does not fall under the other.