SLA times dont cover our work hours

We cover a few time zones and have colleagues in the US. Our UK engineers clock off at 5.30pm, and the US engineers continue til their 5.30pm (our 11.30pm UK time).So if a P1 comes in, we dont offer 24x7 support at the moment so our support hours are 9am til 11.30pm UK time (4.30am - 5.30pm US time). So I can set a customer to a default time zone of GMT put the P1 on 24hrs, tick exclude non working days but the time options in the drop down dont show out support staff coverage hours...... Is there a was we can customize this to combine the UK and US hours?