Is it possible to use a stock, ETF or indexes as an indicator, in order to scan for stocks etc. having relative performance to such stock or index? 

You can add an indicator Add Plot to Chart (the green + in the upper left of the chart) and just start typing the symbol. Once you see it in the list, you can select it.

You can also add another symbol using the comparison symbol feature.

But if you want to actually scan, you would probably want to use a relative strength indicator (just plain RS, not RSI). Then add a Rate of Change Percent indicator to the chart and change its data source to use the RS indicator instead of price.

You would then be able to click on the ROC% indicator and select Create Condition to create conditions based on how much the active symbol over or under performed the comparison symbol over the period of the ROC% indicator.