You will need a third party application called LoopBack to be able to route the audio from Skype to Studio One: https://rogueamoeba.com/loopback/
With LoopBack and a USB Microphone on OS X, you will need to merge all these devices together by creating an Aggregate Device (https://support.apple.com/en-us/HT202000
Go to Audio Midi Setup in OS X, this is found in your Applications > Utilities folder. Set up the Aggregate Device using LoopBack, your audio interface, and your USB microphone.
In Skype, you will have to change audio out to LoopBack (But may need to select the Aggregate here). Skypes audio output will then run through the Aggregate Device.
In StudioOne, you will set Preferences > Audio Setup to the Aggregate device you created in AudioMidi Setup.
Once you do this, you will be able to add a track in Studio One fed from Skype. Also, you can assign the USB mike to a track.