To shift to a litter box....
Establish a regular place where the pee pad goes. Get a litter box, put a used potty pad in the bottom, place in the usual place. Be sure it's been used so the smell will be a trigger, and in the usual place. Put doglet in at the usual potty time. After a few times, add some straw....there is something about it they cannot resist. Once comfortable going on straw, remove pee pad. Then put litter under the straw and then phase out the straw and you'll have a litter-loving dog in no time.
If you wanted to have a covered box, use the bottom part for the first bit of training, get the behavior well established, then add the top.
Make location changes slowly, a couple of feet per day, if needed.
In my sun room I have a splash pool or two, depending on what's going on, with wood shavings covered in straw. All of them use the potty pool when they are shut inside or if we aren't going out in the weather at night. The small ones will come inside from the yard to use them. Rainy night? No problem! Middle of the night potty run when the 'yotes are howling? No worries! Gotta love it.
susan
Fuzzy Wuzzy was a wabbit, Fuzzy Wuzzy had a dandelion habit! RIP little Wuz... don't go far.