If you'd like to transfer voice memo M4A files from your Mac or PC to the Voice Memos app of your iPhone or iPad, iExplorer can certainly help you do that.
The Media Library section of iExplorer is read-only and is only for transfer voice memos, music, etc. to your computer, but you can go into the Media folder instead (located in the left panel as well) which is a file system view and double-click on the Recordings subfolder to open it. Then, just drag/drop any M4A files into the Recordings folder or right-click in the folder and choose Add Files. Once you've transferred the M4A files you want, now just power off and power back on your iPhone or iPad, and you'll see the new audio recordings appear in the Voice Memos app.