No, there is no program or site that can directly convert an SMX file to an SP file. SMX files are compiled SourceMod plugins, while SP files are SourcePawn scripts, which are the source code for SourceMod plugins. To convert an SMX file to an SP file, you would need to decompile the SMX file back into its original SourcePawn source code.
Decompilation of SMX files is not always accurate and may result in loss of information or errors. Additionally, decompiling someone else's code without permission is often a violation of copyright law.
It is recommended to find the original SP source code for the plugin if it is available, or to contact the plugin author to obtain a copy.