Hi Sander,
Unfortunately I wasn't able to find how to disable entry validation for the Person Picker control. But I found a different solution in case if a person doesn't exist in AD.
I used a text box next to the Person Picker control. This text box was disable initially while Person Picker control was enabled. If a person wasn't found using Person Picker control I enabled text box and user was able to enter the name in the same format as in the Person Picker control.
I know it's not the best solution, but after spending countless hours to do the right way, I decided to do the way I described above.
Please let me know if you find the solution.
Best regards.