Altium doesn't have an equivalent to net alias. It does allow the use of a "Net Tie" which is a two pin component with a wire between the two pins. This specially defined symbol/component lets you connect two different named nets together without creating compiler or DRC errors. You can define it as a physical or just logical component by specifying BOM or non-BOM as the type.
I generally use two short length pins back-to-back. You set the Net Tie property in the schematic symbol and/or PCB footprint properties dialog where it says "type". It's mentioned in the help document TR0111.
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