With HP in order to keep the System, Recovery and OEM Diagnostics tools partitions bootable, it is best to convert C to Logical so that you can add as many adjacent Logical partitions as desired.
As you discovered converting to Dynamic is not an option.
In addition, in order to force it to convert back to Basic using Easeus Home Edition or (better yet) Partition Wizard bootable CD, you'd need to first use the tool to delete the extra partitions you created which tipped it to Dynamic against the warning.