As other suggest, it is unlikely.
Most WAPs run at full power and need to radiate that in every direction. You could find one with an external antenna connection, fit it it the loft and choose an antenna with a hemispherical coverage pattern - that might gain a couple of dB but might create more blackspots.
You could get one with three fixed antennas and tweak those around on the chance it might work, but it might not.
You could get two "identical" WAPs and find that one has great coverage and the other is no where near as good. There can be 1 or 2 dB difference in output power.
Why not get a second WAP - you already have Ethernet around the house so fitting a second would be easy and if loss of a socket is a problem wthere are WAPs which also act as small switches with 2, 3 or 4 additional posts.
In my house, I am currently running three WAPs - and a fourth will be going in this week.
M H C
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