No you can not. RAM is limited and cannot be increased as far as I know.
If you have that much RAM on your PC, why don't you just host a server yourself, on your own PC, by Port-Forwarding the router? This will likely solve the lag-issue, If you got the ressources to do it.
Anyway, If you want to avoid the aternos server overloading, just tell your friends not to explore the world. Generating new Chunks is the main (and mostly the only) reason for server lag spikes. Just generate as few NEW chunks as possible. Moving around on already explored terrain will not cause much lag. (You could also download the world, explore everything in a big radius, and then reupload the world. So then all chunks are pre-generated, no new chunks will be generated when playing on the server, and thus lag will decrease)
Thanks for this, how do I Port-Forward it?