'Extrapolability' is an existing English Word. My print copy of A Supplement to the Oxford English Dictionary (1972) does not, however list it. I am indebted to EL&U member Janus Bahs Jacquet who drew my attention to the presence of the word in the online version of the OED, as below:
[f. extrapolate v. + -able; cf. interpolable a.]
Capable of being extrapolated. Also exˈtrapolatable a.
1940 A. J. Ayer Found. Empirical Knowl. iv. 224 Correlating the sense-datum with other sense-data, in such a way as to make its occurrence inferrible by the use of extrapolable laws. 1956 J. S. Bruner et al. Study of Thinking iii. 56 It makes a vast difference in behavior whether he is ‘set’ to find out the extrapolatable properties of the class of people who are influential. 1964 Amer. Philos. Q. I. 264/2 This merely makes the word ‘extrapolable’ bear the whole unanalyzed problem of induction. 1984 Oil & Gas Jrnl. 9 Apr. 88/2 This..law was..determined by experiments carried out under quite different conditions, thus guaranteeing its extrapolable nature on the Bousseus test loop. 1985 Ibid. 11 Feb. 84/1 There is no reason why extrapolatable data cannot be used.
Hence exˌtrapolaˈbility n., the quality or state of being extrapolable.
1968 Science 1 Mar. 935/1 Obviously the degree of extrapolability from evidence or crafts extant today must vary enormously from case to case.
A Google Ngram suggests that the word may have been in use prior to the first quoted example in the OED:
The earliest link from Google Books is dated 1930:
-from Contributions From The Physical Laboratory Of The University Of Michigan, Volume 6. A current day link to the 1930 paper is here: http://journals.aps.org/pr/abstract/10.1103/PhysRev.35.299
Other documents from the 1950's and 1960's containing this word can be found at this link: http://tinyurl.com/extrapolability