Camillo -
If I understand your question correctly, it seems that you are looking to use the properties tab and "rarity" field to make any item in the game look as though it is a set piece (turn the border/background green). There is no option that I know of in properties to make that happen and nothing in the flags section either at this time. It may be possible that you could search that massive amount of options for the flags section (i.e., entering your own numbers) and discover that you can make any piece become green. However, that will take quite some time.
Note that you can make any set piece in the game with the editor. You simply need to create a new item, choose the set piece you want and then not change the rarity field in the properties tab. The item should have the standard "set" green background.
I hope this helps.
If I understand your question correctly, it seems that you are looking to use the properties tab and "rarity" field to make any item in the game look as though it is a set piece (turn the border/background green). There is no option that I know of in properties to make that happen and nothing in the flags section either at this time. It may be possible that you could search that massive amount of options for the flags section (i.e., entering your own numbers) and discover that you can make any piece become green. However, that will take quite some time.
Note that you can make any set piece in the game with the editor. You simply need to create a new item, choose the set piece you want and then not change the rarity field in the properties tab. The item should have the standard "set" green background.
I hope this helps.