The whole point of the limited mutations in thrive is that you must build on your preexisting species so there is no single point in the game where you are allowed to drag and drop premade parts. The whole thing is a smooth continuum starting with your cell and ending once you are tribal and can’t make even the small tweaks anymore. So you first start placing individual specialized cells and eventually you are adding tissue to different organs and slightly editing their size and placement. So premade parts are very incompatible with the concept of smooth evolution that thrive aims to have (there’s not going to be stage “transitions” as such like in spore instead it will be all smooth).