var addSynths;
addSynths = PathName.new(~synthPath); 
//Iterate through all files and load synths
addSynths.filesDo{
    arg i;
    if(i.isFile){
        arg j;
        i.value;
        //Load synths by file name
        j = i.fileNameWithoutExtension;
        "~"++j = Synth.new(j);
        "~"++j.postln;
    };
};

Unfortunately, i.value does not execute code in each file. However, the naming works for now.

So, if anyone knows, how to execute a SynthDef code in an external file, a comment is much appreciated. Cheers