The time frame mechanism is for starting and ending a bar by a user defined event at a multiple of the bar period. This is not totally trivial, but you can find examples in the manual.

A portfolio system is a system that trades many assets with the same script. Since there is only one bar period per script, a portfolio system can not be based on renko bars that would require different bars for any asset. You could only emulate renko bars with time frames. I have no ready example of this, though.