I have hatched Many of eggs in a still air incubator----usually 70 to 80%, has been lower and higher. I did this NEVER using a hydrometer/humidity meter---did not own one for years. I just filled the smaller water reservior for the first 18 days, then both of the reserviors the last 3 days adding a small wet sponge if I felt it was needed. With the still air I keep the temp around 101. I think the biggest thing that helps me have a good hatch is when I close the incubator on day 18---after I candle the eggs, remove the turner and fill both reservoirs is--I DO NOT open the incubator again for NOTHING till all the eggs hatch or day 22---which ever comes first! If I need to add water I do it with a flex straw through a vent hole. If a Chick looks like it needs Help----If it will hang in there till all hatch or day 22---I will help it---Nothing can make me open it before. I do the same in my Big Hatcher and I usually hatch close 100%.
You can put the eggs into another incubator while you add the fan kit---should work fine. I have added fans to my styrofoam incubators too but I always used a small fan I had laying around and a lower voltage power pack(looks like a cell phone charger)--But I have alot of "stuff" laying around! If you have questions, just ask--BUT ask before you do it is best---plenty of people here to help you. Good Luck!!