While calculating occasional stresses we separate out the effect of occasional loading such as Wind or psv force. It is then added to sustained case using the scalar combination. My question is if it should be added to hot sustained case which is the actual sustained case at operating position of the pipework (there could be a support lift off).

Any idea???

Hot sustained stress or alternate sustained stress (Alt-SUS) provides another look at the force-based load distribution based on an operating support configuration. B31.3 para. 302.3.5(d) states that the SL used in the allowable expansion stress range (SA) in eqn. (1b) should be the maximum SL "considering all support conditions".
So we may have several SL's from which to choose. For (1b), the maximum SL can come from the installed case, or operating case 1, or operating case 2, and so on (perhaps even operating plus occasional). It is a statistic rather than a stress based on a single system state.
CAESAR II automatically selects the maximum SL from all SUS stress types. I suggest you use this same approach for your sustained plus occasional checks. I would create a new load case combination which includes all sustained stress calculations with a combination method of MAX. Then, for the sustained+occasional combinations, add your occasional loads to this (maximum) sustained case. If some nodes fail the stress check, only then would I attempt to exclude those high sustained that cannot coexist with the occasional case. (Here's an example for a relief discharge evaluation: if the installed support configuration produces a high sustained stress, that stress need NOT be summed with the occasional (discharge) stress since you can't pop a relief valve on a "cold" system.)