Humbrol filler, Squadron Green Putty, Mr Surfacer, Mr Dissolved Putty, cellulose thinners, acetone, and liquid glue can all be mixed with each other to get the consistency you wish
I have a jar of Mr Surfacer to which I've added Green Putty when I've squeezed a bit to much out
No matter how carefully you replace the top, I find that tube filler gradually dries out and becomes too stiff to work or apply. I squezzed & cut open the tube & contents into a small jar and thinned it with cellulose thinners and the dregs of some liquid glue
Instead of cleaning a brush or using a disposable one I scrounged a more or less empty nail polish jar and use the brush in that. I cut the bristles a bit shorter so they don't flop around too much. Then refit into the bottle; this acts as a keeper so the brush doesn't dry out & harden
Don't worry if a bit of nail varnish gets into the filler, it'll all mix in
Search YouTube for 'plastic modelling filler' and you'll see the sorts of mixtures that can be used