How to add an existing folder with files to SVN?

If I correctly understood your use case, I suggest to try using svn add to put the new folder under version, see here. The following will add the new folder with files recursively under version control (if you are inside valid working copy):

svn add new_folder
svn commit -m "Add New folder to the project"

If you are not in a working copy, create it with svn checkout, copy new_folder there and do the above steps.

OR

Try svn import, see here; the following will create a new folder and upload files to the repository:

svn import -m "Import new folder to the project" new_folder \
        http://SVN_REPO/repos/trunk/new_folder

Also note that:

After importing data, note that the original tree is not under version control. To start working, you still need to svn checkout a fresh working copy of the tree

Let's say I have code in the directory ~/local_dir/myNewApp, and I want to put it under 'https://svn.host/existing_path/myNewApp' (while being able to ignore some binaries, vendor libraries, etc.).

1. Create an empty folder in the repository svn mkdir https://svn.host/existing_path/myNewApp
2. Go to the parent directory of the project, cd ~/local_dir
3. Check out the empty directory over your local folder. Don't be afraid - the files you have locally will not be deleted. svn co https://svn.host/existing_path/myNewApp. If your folder has a different name locally than in the repository, you must specify it as an additional argument.
4. You can see that svn st will now show all your files as ?, which means that they are not currently under revision control
5. Perform svn add on files you want to add to the repository, and add others to svn:ignore. You may find some useful options with svn help add, for example --parents or --depth empty, when you want selectively add only some files/folders.
6. Commit with svn ci