"if exceptionally" is correct and usable in written English.
You can use it to indicate that something only happens in rare or unusual circumstances. For example, "If exceptionally heavy rains occur, the river may overflow its banks."
She appears to be a perfectly ordinary, if exceptionally lovely, Buenos Aires teenager.
If exceptionally a board decides that a chief executive should become chairman, the board should consult major shareholders in advance and should set out its reasons to shareholders at the time of the appointment and in the next annual report".
China is unique because it combines standard-issue (if exceptionally cynical) mercantilism with other policies, like forced savings and systematic technology acquisition, made possible by its despotic ex-Marxist political system.
A new Prison Rule that no establishment should hold more prisoners that is provided for it its certified normal level of accommodation, with provisions for Parliament to be informed if exceptionally there is to be a material departure from that rule.
If, exceptionally, a UK hospital from outside the study wishes to refer a patient, the transport team from the ECMO centre will go to the hospital and assess the patient.
If an exceptionally beautiful person became a computer programmer she or he would forgo a higher salary that could be had from modelling.
Organized by José Roca, an independent curator, the exhibition looks as if an exceptionally creative sociology professor had displayed audio-visual materials for an introductory college course on the systems and semiotics of social control.
We've just described Mr. Romney's dream scenario, an imaginable (if not exceptionally likely) sequence of events that could have the race looking almost even by a week or two from now.
He's an old, mean joke with a new face, in other words, if an exceptionally profane and unpleasant one.
In view of the δ13C δ15N results and the inherent interpretation difficulties, it is not possible to completely exclude that skull A can be from the 14th century AD, if an exceptionally high reservoir effect is taken into account.
Even more frustrating has been the modest if not exceptionally low effect sizes associated with the genetic variants that have been identified in most GWASs (6, 7).
