Even if its "baked into" the kernel file ... it's just a (z)cat file which can be devided into its parts. But without the buildscript you have to search for the file headers for splitting "by hand".
For me its not 100% clear what the threadstarter wants to achive. Are you building an image with openwrt sources? Do you have a build which you want to modify? If you build from sources there should be a initramfs config file which you can edit and target to the location where initramfs has to be loaded (there are several ways if you google). If you want to edit a given kernel imagefile then you have to extract the kernel file, split it, extract the parts (kernel and initramfs), make the changes and do the things backwards. But I would not recommend that if you want to change things inside kernel itself. Because you cannot just edit the kernel image (its a binary) like the initramfs. But for starters I've googled fast: https://blog.packagecloud.io/eng/2016/03/08/how-to-extract-and-disassmble-a-linux-kernel-image-vmlinuz/