Would I be able to access myUser.dmg that created with Disk Utility after re-installing OS X? I am going to format my OS X Machine (El Capitan 10.11.6). 

I created manually a disk image file (.dmg) of /myUser folder on an external HDD using Disk Utility.

Will I be able to access/mount this disk image (.dmg) file after re-installing OS X or am I going to have problems with permissions and etc ?

Disk image file had read/write format (not compressed) with no encryption.
Your problem will not be mounting or accessing the .dmg file, as you can always set any drive except your current boot drive to 'Ignore Ownership' from Get Info on that partition.

Where I foresee difficulty is in re-integrating that extracted folder into your new OS. I'm not too sure the system will let you simply overwrite a lot of the data in your new User folder.

I'm sure the ideal solution would be to do it the recognised way, using a Time Machine backup.