Is it possible to use global net names in Instance sheets of the blocks ?
No, not with an instance value, if you need different global supplies for multiple instances of the same block then you need to bring the net up to the top level of the hierarchy and add the global signal there.
This means it is not possible to access or use the net of one block into another block. Then the only way is to define global symbol which are especial symbols for example like power supply or ground connectors, right ?
It depends on your use case. If you have two identical blocks (two instances of the same schematic) and you want one to have for example 3.3V as a global and the other to have 3V3 then no. If they both use the same global then yes you can use globals.
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