it sounds like you are describing a "freeze" function rather than "loop interpolation" - which are related but raise different questions. First off, look at the helpfile for the [grooveduck] object referred to above. If this does not do what you want, then you are probably looking for a freezing or granular solution.
You're probably aware that in looping we skip from the end of a sample-buffer selection back to the start of the selection. Where the samples differ there will be discontinuity clicks. With longer loops (say >100ms) you will *always* hear the point of interpolation, unless you do some overlapping:
in much the same way you are overlapping posts on the same subject - there will be an offset delay between copies of replies to each ;p
Granular playback/synthesis (there is a distinction NB) uses much smaller loop sizes (c. 5 ~ 50ms) and multiple copies/overlapping. If you are (like me) a relative newcomer to MaxMSP, then this is not trivial to achieve - it will take a bit of reading and practice. There are however many many examples of excellent granulators out there: Robert Henke, Timo Rozendal's bloody excellent grainstretch~ external, SugarSynth, Nathan Wolek, N. Sakonda. . . . and Max ships with its own usable granular engine (look in the /examples folder).
Secondly, as previously stated above, you could try pfft-freezing, which can sound very sexy indeed; as a semi-n00b I only know of one: