If I borrow money on a credit line, which I lent to another, and when this other repays me , am I allowed to accept also the interest charged to me by the bank, for the loan?
Your loan from the non-Jewish credit company is your loan and you can take that out with interest and repay them interest.
Your loan to your Jewish friend is a separate transaction and must either be interest free or done with a heter iska.
You can ask your Jewish friend to pay all fees and charges you incurred in taking out the loan but not the interest charges for the loan. Were you to do that, in effect your loan to your Jewish friend is a loan with interest and the way that you are asking him to pay you is by paying your own debt to the credit institution that lent you the money.