I’m not sure I completely understand the question, but the thing to understand is the extra transmitter is exactly the same as the ISS transmitter. The limitation is only what can be received by the console/WLL.
For the console, you can only receive 1 ISS and you can use the extra transmitter for only one sensor at a time (ie a temp/hum sensor or wind sensor). Additionally, as far as I know, even though the console allows you to select an additional rain sensor, the console will not receive an additional rain sensor beyond the 1 ISS transmitter.
I don’t have or know WLL, but my understanding is you can connect multiple ISSs, so your solution would be to set it up as a second ISS and connect as many sensors as you like.