It is possible converting strings or characters to lowercase? How?
It depends on what version you are using.
v4.0beta
[admin@MikroTik] > [:lua "io.write(string.lower('HeLLo\\n'))" ]
hello
Thank you!
3.xx on all routerboards 
in v3.x you won’t find such commands.
Only option is to write your own upper to lower string converter.