Hi,
Is there any way to add a Multi Select functionality in the Plasmic Select component?
Not at the moment 
Any recommended way to design a custom one using Plume hooks.
Also, do we have any tutorial or guide on how the existing Select component is designed?
I managed to create one using downshift library.
Plasmic + Headless UI Libraries 
Yeah, we ourselves make use of downshift a lot with our Plasmic-designed components. Plasmic fits really well with headless UI, like downshift and react-aria 