According to an agent at their Harambee Avenue office, this doesn’t work that way. Bundle extension only works if you buy an equal or greater amount. This means the 90bday one would expire on due date and your daily bundle expires the next day. Same logic is used with the ‘bonus’ bundles telcos give you.
I suspect the codes used to purchase the bundles are different and what happens is if you buy different bundles, they simply stack all of them together like a strawberry vanilla pistachio Sundae at a restaurant and you consume each desperately instead of mixing them all up.
The ‘Why do data bundles expire’ article should help explain this.