How To Top Off An M1 Garand Magazine...

A loaded M1 en bloc clip...yes, that is a clip

Yes it can be done with a lil manipulation...

The following written instructions are from a no longer available online article entitled "My M1 Speaks For Me!"

"...there are two ways to accomplish "topping-off". The obvious way is to eject a partially-exhausted clip and reload with a full one. The other is to lock the bolt back and insert loose rounds into the clip in the weapon, although, I don't think WWII M1 riflemen often had loose rounds in their pockets! I practiced (as all sensible rifleman who depend on their weapon for living in a battlefield environment did) and can now easily add two or three rounds to a partially emptied en bloc in the Garand:

• Open the bolt, catching the ejected un-fired round, while pressing down on the bolt lock stud to the left of the magazine well. If the barrel is hot, wait for it to cool to prevent a 'cookoff.' If the clip ejects, press it into the magazine well.
• With the bolt held back, press rounds into the partially emptied clip with a downward, rotating motion until the round snaps into place. Avoid pressing down on the clip itself.
• With the knife edge of your right hand holding the operating rod handle to the rear, press down on the clip to release the bolt lock.
• Slowly raise the right hand, seeing that the bolt engages the top round, and swing the right hand up and to the right to release the bolt to chamber the top round.

In this way it is easy to top-up to 7 rounds. The eighth would be difficult, I grant you, but then you should have just ejected the partial and loaded a full clip in the first place!"

Here are two variations, the first (left) following the above method, the second (right)  a slightly different method in that he keeps the clip seated in the magazine and holds the bolt open...

Here's an excerpt from the 1965 edition of the manual: FM 23-5 US Rifle Cal .30 M1 - Operation and Functioning