You could use the Apple Loop Utility (ALP) to convert them to Apple Loops....which will then cause the loops to do as you have asked.. much like any other Apple Loop
The ALP comes with Logic Studio (It can be found usually in Applications/Utility or Applications/Other ) or if you downloaded Logic from the App store then you can obtain the ALP app by going to the developer's website.
You can register for free and then you can download it.
Another solution: Use Flex
2 basic methods:
a) Import the audio loop. Calculate the tempo (use Options Tempo:Adjust tempo using Beat Detection (sometimes Ok), or 'manually' by setting the region to exactly the right length, the cycle to match, the Options:Tempo: Adjust Tempo using Region Length and Locators). In both cases you are temporarily setting the Project tempo to match that of the loop.
Then enable Flex for that track (and make sure that Flex is enabled for the loop region).
You should now be able to change the project tempo to you heart's content.
b) Ignore the above and do the whole thing manually:
Import the loop to an Arrange track. Count how many bars/beats the loop is. Enable Flex for that track. When Flex is enabled, you can then Option+Drag the end of the region to timestretch/shrink to the required length.
You may need to try out different Flex Modes to establish the best one for the particular content.
Ah, the Apple Loops Utility! I haven't pulled that one out in years. Okay, so I spent the past day trying to get these drums loops to follow tempo, but still haven't found the trick. I've opened them all with the Apple Loops Utility and saved them to a separate folder (with transient markers set). Still no luck.
Yes, the Flex Mode approach should work for individual projects. I was hoping to audition loops from the loop browser. If I find a better solution I will post it.