It's unclear if the solution for which you're asking (how to convert primary partitions into extended partitions) is the appropriate one for your system, or if it's even possible on your system.
/boot/efi mount point is generally used by the EFI System Partition (ESP), which in turn is used only on EFI-based computers. Thus, it sounds as if you've probably got an EFI-based computer, although that's not 100% certain based on the evidence you've presented.
As oldfred says in his comment to your question, EFI-based computers generally boot with GPT disks, and GPT doesn't have the primary/extended/logical partition types; there are just 128 (by default) partitions on a disk. Thus, if you've got a GPT disk, it's impossible to convert from primary to logical partitions, since your existing partitions are neither primary nor logical (although they're more like MBR primary partitions in most respects), and GPT doesn't support logical partitions.
OTOH, if the
/boot/efi issue is a red herring, you might have an MBR disk. In this case, you can use FixParts, which is installed as part of the
gdisk package in Ubuntu, to convert from primary to logical, with some caveats, as described on the FixParts Web page. Even in this case, though, it's not clear that converting primary to logical or vice-versa is really the best thing to do; a better description of the problem is necessary to offer advice.
If you want a better answer, you'll have to post more information. The output of
sudo parted -l, as requested by oldfred, is a starting point. Still more information can be summarized by the Boot Info Script; run it and post a link to the
RESULTS.txt file that it generates. Also, please describe precisely where you are and what's going wrong -- for instance, have you successfully installed Kubuntu but are having problems booting it, or is the installer failing, or are you able to boot but just get an error message during the boot process?