You can merge fontello collections using a text editor by opening the config.json file of one collection and copying the glyphs to the config.json file of the other collection. After you do that, drag (upload) the new merged config.json file to fontello.com and you can then download your new merged collection.
To add new icons, simply drag (upload) the SVG images to fontello.com. In this case you may need fix the paths (fontello will only accept files with a single path) within your SVG files before uploading them. You can do this a free tool called Inkscape by following these steps:
- Open file
- Select all
- Object -> Ungroup
- Path -> Union
- Path -> Combine
- File -> Vacuum Defs
- Save as -> Plain SVG
If the SVG file has multiple paths that you cannot remove by combining paths, you can use Edit > XMP Editor to remove them. You should only have one path when you are done.
You can also check the svg file output in a text editor. If saved correctly, you should see a single element and an empty element.