In the cases of the following FORCE words, do the compounds agree with the base forms in having the same sound [or]: force/forceful, sport/sporty and support/supporter?
If Russ is a native speaker of English, why is he asking? (He should say the words aloud and find out.) If he is not, the answer is very simple: yes, they agree with the base forms. The suffixes -ful, -y, -er do not normally trigger any vowel change in the stem to which they are attached.
Whether the sound is or or something else depends on the kind of English you speak. In RP and similar accents these words all have ɔː, thus fɔːs, ˈfɔːsfl̩, spɔːt, ˈspɔːti, səˈpɔːt, səˈpɔːtə. In Scottish English, and for some Americans and others, yes, they have or, thus fors, ˈforsfl̩ etc. For other Americans, of course, they have ɔr.
But there are certain other suffixes which do indeed trigger a vowel change (for some of us). Historically, this is the alternation that we see in pairs such as tone–tonic, episode–episodic, verbose–verbosity, where a stem with the GOAT vowel switches to the LOT vowel when the triggering suffix is attached. (We have the same “trisyllabic laxing” with other pairs of vowels in serene–serenity, divine–divinity, profane–profanity and so on, as students of Chomsky and Halle’s Sound Pattern of English know.)
Since historical GOAT plus r becomes FORCE, in stems ending in r the alternation is instead between FORCE and LOT. We see it in pairs such as historian–historical, floral–florist, explore–exploratory, in which in RP and most accents of England there is a vowel difference ɔː–ɒ, thus (h)ɪˈstɔːriən, (h)ɪˈstɒrɪkl̩, ˈflɔːrəl, ˈflɒrɪst, ɪkˈsplɔː, ɪkˈsplɒrətri. But others may have lost this alternation.