Copy the actual devices in the schematic without any connection then place them in layout.
You can make them all pins are short circuited and they may share same active layer even DTI region.
You mean to say I put first the dummy I want in the schematic without connection, then for this the schematic will complain about it so I need to make the simulator ignore it. Now this instance will be visible to Layout where I can use it.. looks easy and indirect method.
However there is also direct tool from Layout XL where it is special to genertae dummy instance without interfering the schematic. One of these tools is ModGen and also other tools provided by Layout XL which I don't know
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for example this vidio expalain how to connect dummies but not showing how to creat them