You could very easily dilute your cell supernatants with RPMI. If you do so, you need to use the sample equivalent amount of RPMI as your assay blank too, and subtract this value from the sample readings.
Alternatively, you can dilute samples with the assay diluent that you normally use for the initial blocking and for diluting standards, etc. Then your blank can be the assay diluent alone.
You will have to figure out the extent of dilution such that your values are within the linear range of your standard graph (we generally use 1:5 dilution for lymphocyte sups).
RPMI should work well. Mukta is right about diluting standards in the same way. In fact, including fetal bovine serum might be a good idea as well, because it could decrease background binding.