Altium: How can i add thermal releif to the GND footprint of one component?
Yes. In the design rules go to Polygon Connect Style and use a query like this:

InComponent(&lt;component with pads in question&gt;) AND InNet('GND')

and set it to relief connect. Put this rule at the top of the priorities, and in the default rule simply have "Direct connect" set.

Relief connect rule (top priority):


"U10" is the component with ground pads that should be relief connect.

Direct connect rule (default, lower priority):



I believe that would work the way you need it to.