Are there any analogue devices that can be connected to synthesizers like yamaha psr S550b to modulate sounds and get an output back from my synth's speakers? (Basically I want to minimize the use softwares) Currently I have no idea about how modulation works but only how turning the knobs change the sound output..
No, with keyboards like the psr s550b this is not possible. Those devices are usually "closed", in terms that they do not offer any effects channels or similar.
However, you are free to use effects after your psr s550, by connecting the output to any effects device (e.g. a reverb or delay effect) and send this to speakers/mixer/etc.
The only usual way to affect sounds during the actual sound creation pipeline I can think of are analog synthesizers. With those you can influence control voltages and patch them to different inputs/outputs.
With the usual consumer devices by Korg, Yamaha, Roland etc. you have digital sample/ROM based sound production and those try to bring a full range of effects, but even those do not offer to loop in external effects. Those always have to be "downstream" after the sound is created.
edit: I just noticed that you just said "modulate". Maybe you could be more specific, this is a really big subject.