W2W does not currently provide a way for an employee to pick up, or trade, partial shifts from the Tradeboard.
For now, perhaps your employees can send a message to their manager indicating they can work only a portion of the shift, and the manager can decide if the unassigned shift should be edited to match their availability, or if another shift can be added to fill the missed portion of that shift.
W2W allows you to schedule employees to as many shifts per day as you like, but you cannot assign a portion of an existing shift already on the schedule. To do this, you would need to create two shifts by editing the existing shift’s start or end time to reflect the shorter shift you want to assign, and if you then needed the remainder of the original shift time covered as well, you would have to add a new shift with start and end times that covered the remaining time the partial shift missed. →More on quickly splitting shifts
Note that if you wish to offer your employees the ability to set their own times, you can go to SETTINGS > Global Employee Permissions and allow some or all of your employees to edit shifts (Permission 4). They could then be allowed to edit a shift’s start or end times without manager assistance. You can allow this for all employees, or you can offer this ability only to select, trusted employees.
Partial trade off tradeboard, pickup cover others for a couple of hours without actually swapping shifts