Does an external interrupt interrupt ADC or does ADC stall interrupt?
No, the interrupt occurring has no effect on the A/D.  The A/D runs from the instruction clock or its own clock, depending on how you set it up.  Both these keep going during a interrupt unless you deliberatly execute a SLEEP instruction to stop the processor clock.

At most, the 1 Hz interrupt could delay the interrupt routine processing the A/D conversion done, if you are doing this by using interrupts at all.  There is no requirement that A/D results be handled by using interrupts.  Even if so, the A/D will perform its conversion and write the result into ADRESH:ADRESL regardless of whether the processor is taking a interrupt or not.  In fact, the processor itself is not really "in" a interrupt.  That's only a software abstration.  When the right conditions are met for a interrupt, the processor executes a call to location 4 and turns off the GIE bit in INTCON.  That's all.  The rest is up to firmware.

Once the A/D has finished a conversion, the result will be available in ADRESH:ADRESL.  It is up to the firmware what to do with that.  The value will stay there until a new conversion is completed.  If the firmware doesn't start a new conversion until reading the result of the previous, then nothing can be lost.  If a new conversion is started automatically, then it is possible for the previous data to be lost if the firmware didn't get around to reading ADRESH:ADRESL in time.  Look at the acquisition and conversion time of the A/D and the instruction rate, and you will see there are generally plenty of instruction cycles to grab a conversion result, even if a new conversion is started automatically by the special event trigger of a CCP module.  Just make sure the clock tick interrupt code is not so long that it takes longer than it takes the A/D to make a new result.  This should be easy.  Any lengthy processing is best handled by setting a flag and letting the foreground code get around to it when it gets around to it.  The interrupt code should handle only the immediate servicing of the device.