Using LE's as additional SRAM

You could make a grouping of LEs behave like memory but there is no switch that you can flick to turn your device into a big MRAM if that's what you are asking (that would be pretty cool though). The embedded memory in an FPGA is SRAM with some optimized timing, but an LE on the other hand is a lookup table and a register (so it's not memory cell based like the embedded memory is). If you really want to try this you can instantiate a memory from the megacore and I think you have the option to create it out of LEs (the setting will have EABs in the name but I forget the actual text). So you can do this but you might (probably will) degrade the performance of your system.