Hi, you can use the lower left corner button, 'New content', then fill the form and press save.
Actually you can find the 'total conversions' as a mods category.
And, if you want to create your own presets it is very simple after having made a selection of mods, addons and/or maps, you just need to access to selected files panel and use the button in the lower right corner to save the selection as a preset.
The preset configuration window will be displayed, where you can add a title/name and select the type of configuration (category), you can also modify the selection made and add a custom image or select one by default.
As configuration type or category you can write any value and this will create an automatic filter in the presets panel.
You have all this information on help panel Use F1 or last right button on main toolbar to access the help panel.