Hello Florain
The FS is a FS only if it is not in the routing table. If it is in the routing table it has the best metric and is thus considered a successor, even if there are more than one such as is the case with EIGRP load balancing.
If you do unequal cost LB, then the routes in the routing table are also considered successors and not feasable successors. But putting aside terminology, the answer is yes, the routes that were FS (at least the next one(s) in line) is inserted into the routing table and becomes a successor.
The deciding factor of choosing the next successor is always the same, regardless of whether or not you are using load balancing. EIGRP will keep up to six feasible successors in the topology table and only the one (or ones) with the best metric (or metrics) become the successor(s) and are placed in the routing table.
I hope this has been helpful!
Laz