W2H does not currently provide a way for a helper to pick up, or trade, partial shifts from the Tradeboard.
For now, perhaps your helpers can send a message to their coordinator indicating they can work only a portion of the shift, and the coordinator 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.
W2H allows you to schedule helpers 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 helpers the ability to set their own times, you can go to SETTINGS > Global Helper Permissions and allow some or all of your helpers to edit shifts (Permission 4). They could then be allowed to edit a shift’s start or end times without coordinator assistance. You can allow this for all helpers, or you can offer this ability only to select, trusted helpers.
Partial trade off tradeboard, pickup cover others for a couple of hours without actually swapping shifts