I don't know if you can do this with MOBI or AZW3, but you sure can with KFX. If you decide to use the KFX format, these are the steps to add page count:
1) Install KFX output plugin. (Preferences > Plugins > Get new plugins).
2) Now, on the Convert Books dialog, you will have KFX as an output option. Choose it.
3) Still on the Convert Books dialog, go to KFX OUTPUT icon on the left column. Mark the option "Create approximate page numbers".
4) If you have the "real" page number saved on a custom column, you can choose it on the option "Lookup name of custom column with desired number of pages". In this case, the plugin will use this number instead of calculating the approximate number.
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