It's not as easy as it seams.
Currently you use the friction shifters on the down tube.
You can move them to the stem:
You can buy a pair of bar end shifters:
I don't know if integrated brake/shifters from a mountain group will work with your old derailleurs.
Though, derailleurs can be found cheap.
You should consider the flat bar diameter is to be compatible with your stem.
The cheapest method is to buy a tall stem like Nitto Technomic
and add bar end shifters. Also, you need new cables, for bar ends you need longer ones, for tandems.
With the higher handlebars you can ride in a more vertical position.