broody BO; can I plant a day-old chick under her?

my Buff Thelma went broody over the weekend, and she's hogging the only nest box the girls use.  She has been sitting on the eggs - I don't know how the other 6 girls manage to get in the box and lay while she's flattened out in there, but I'm sure she's happy to have 6 warm eggs to sit on.  I'm a little worried about the day she decides to go after me when I have to tip her to collect the eggs.  I was reading about helping her transition out of her broodiness, and love the idea of sneaking a newly hatched chick under her at night when she's sleeping, and she'll wake up thinking her baby hatched!  It's probably still a little too early to do it, since she's only been broody for a couple of days, but I wonder if anyone else has tried this approach.  I don't mind adding one more chick to my flock, but since my existing flock of 7 are only 10 months old, and my coop isn't big enough for more than 8, I don't want an overcrowding situation, and I don't really have room to expand.  any thoughts?

I sent Thelma to solitary confinement yesterday lol - a large dog crate with food and water, and one lowly little golf ball to sit on.  My girls are 39 weeks old today.  They started laying on New Year's Day.  We live in Northern Vermont, so they had quite a bit of cold this winter, and some very short days.  I don't know what made her go broody, but I've heard that Buffs are notorious for it.  I work full-time, so if they haven't laid their eggs before I go to work (at 8:30 or so), they're in the nest box until I get home (5ish).  It was very convenient for Thelma to hop on a pile of warm eggs when she started going broody. This winter I was worried about frozen eggs, but it only happened a couple times.  I put them in the fridge and let them thaw slowly, and they were fine.  If any shells were cracked or not intact, I just scrambled them up for the girls!  Here's a not-so-great picture of Thelma ->  I'll take a few more tonight.  She's usually flattened out like a pancake, and looks like she's in a trance.  She makes the funniest sound when I put my hand in to touch her.  If I can upload a short video, I'll do it tonight!

OMG as I was typing this, my co-worker came in and brought me 11 fertilized eggs to put under Thelma!  She said she would take the babies that hatch if I don't have room for them all.  I am so excited!  I'm going to be a Gram