Knowles, 21, is a central midfielder who moves to Treyew Road from Evo-Stik League Southern Premier Division side Frome Town.
He previously played for Paulton Rovers, Exeter City and had experience of National League South whilst on loan to Weston-super-Mare.
Knowles will feature in the squad at the Whitehawk game this Saturday.
In addition, goalkeeper Tom McHale has been recalled from Toolstation Western League side Willand Rovers as Martin Rice has a damaged finger.