The sense wires of the BMS carry very little current so extending them (within reason) should not pose an issue. It is more important that the connection to the cell be as close to 0 Ohms resistance as possible as the BMS is measuring millivolts . You do not want a "high resistance" in the circuit as that will affect the BMS operation ( LV & HV cut-off plus balancing).
I'd keep them as short as is reasonable and all the same length and make sure that your splice is a good one.
BTW - I consider myself an "un qualified" expert when it comes to Solar stuff. Use my "advice/opinion" at your discretion.
Cheers.