Is it possible to move legendary effects (like "Double damage for targets with full health") to other weapons, like you can do with other modifications?
No it isn't - not without using the console, which is currently only available in the PC version of the game.
If you're using the console, you can bring up the console and select an item before typing in
attachmod and one of the IDs from this page on the Fallout Wikia.
For example, to add Chameleon legendary effect to an item you'd drop the item on the floor, open the console and click on the item and then type
You can't move the effect around to stack effects.
However, if you have another gun with great mods that you wish the effect was on, and that gun has the same general base, then you can just move those mods to the gun with the effects. I do this all the time.
So I have Righteous Authority. I find a sweet laser sniper. I just move the sniper barrel to Righteous Authority, and now I've effectively moved the critical shot effect from Righteous Authority to the laser sniper.