There isn't a real way to do this in RISA.
However, there is a "trick" that can "sort of" allow you to do this.
If you have a number of little members along the length of a larger physical member, then you use the "model merge" command, the program will get rid of the smaller members and only keep the larger one. However, it will take any loading that occurred along the length of the smaller members and apply it to the larger one.
This isn't perfect because you have to be careful with cross sections, material, end releases and such. But, it is good if the main idea is to get the loading from the smaller members.