Manually added steps don't count in the leader board and badge stats, but you may be able to add them in for your own record. It really depends on the activity as some activities give you an option to manually log steps or distance (which changes the steps accordingly) if you log through fitbit. You can log distance with a custom activity, but playing around with it, it doesn't seem to be adding to my steps or distance even though it records the distance I entered. You can manually log the activity for the calorie burn and activity score and that logged activity score applies to your fitbit leader board activity score/active minutes. (It doesn't apply to earndit challenges if you are doing one of them though). I know manually logged walking and running can change step count because I use to have a GPS app linked to my fitbit and it would overwrite my distance (I usually found it annoying and deleted it because it changed my step count). So if the steps are more important to you than the activity, you could log it as jogging or something with something like 1 mile per 2000 steps you want (on average a mile is 2,000 steps but most of us are not exactly "average"). I'd probably just log the Zumba if you weren't wearing it and count those steps lost. I know, if is always the good steps that are lost! But our bodies know the truth! :-)

I don't know if people still care, but I've found the way to add manual steps to Fitbit. Thanks to Justin (Admin, Fitbit),
https://fitbit.uservoice.com/forums/190061-tile-dash-beta/suggestions/3710917-manually-enter-step-amounts

Login to the Fitbit website through a browser: https://www.fitbit.com/
Click on the Log "tab"
Click on the Activities tab
Scroll down to Log Activities
Click on the Walk icon, or type walk into the search field
Set the date, start time and duration
Put the number of steps in the Distance field
Click the drop down menu and select Steps
Click Log

Now go to the MyFitnessPal Check In tab under My Home. You'll see the number of Fitbit steps you entered, if your MFP account is synced with Fitbit.

{happiness}

If adding the steps on the website manually doesn't work, I found another manual way to add them. I accidentally washed my FitBit with my clothes one day because it was attached to my shorts. I still didn't remember that it was in the washed clothes and put them in the dryer. I'm not sure if the washer and/or the dryer added the steps but I had almost none when I put the clothes in the washing machine and by the time I remembered that it was still attached to my shorts it had been in the dryer for about 10 minutes already. Luckily, the FitBit still worked when I got it out! A bonus for me was that I added 6,000 extra steps! Today, I actually forgot to wear it while I was walking around Dragon Con in downtown Atlanta and I figured the mileage to be about 8,000 steps so when I got home I attached it to my shorts and put them in the dryer with a bunch of towels and started it up for about 30 minutes and so far it has added 4,000 steps for me. Of course, I have a choice to dry without heat because I have an older dryer though so I'm not sure if the heat from a regular dryer would kill it or not. I did dry it for 10 minutes last time by accident with the heat on and it survived. I also washed it in the washing machine and it survived so it's a pretty strong little device! Anyway, put it in the dryer for 30 minutes without the heat. You can put it in a sock and put a bunch of towels in there with it to keep it safer. Have fun!