You can do that just fine and pretty easily. All you have to do is get the struct and set members in it and done.
Like this:
To select which members of the struct you want to change, just click on the “set members…” node and on the left side in it’s detail pannel you will be able to select the pins of the variables you want to expose as input. Then you just plug your new variable in it and it will save it to the struct without losing any other variables that are already in the struct.
What this node basically does is internally transfer all the data from the struct to a new version of it except for the variables you changed, those are then integrated into the “newly” formed struct and thus using this node you are able to push updates to a struct very easily.
In our project we use this fo all our customisation and saving, including game options. Everything is updated on the go, exactly the way you want.