is it possible to get 16 days added to existing stamp by local immigration, or must i exit and return? visa expire date is 24june

90 day expire date is april 20th trying to avoid too many pages being used in passport

Short answer is "NO" !

Sorry

The longest extension you can get is 7 days unless you a married to a Thai or have a Thai child which would allow you to apply for a 60 day extension.

thanks ,maybe i will change the flt date