There are may be a several solutions .If your input signal is periodical , lets say sinus , and your ADC sample frequency is much less then input one . So you may sample it for a long time and each your sample will measure the input wave at other place of the period , by some simple algorithm you may build your input signal period back . But it will require a long of time sampling ( to acquire a lot of samples ) and a strongly periodical input.
The other solution is to take a several low frequency ADC converters while sample clock of each one will have phase shift respect to other ((1/max input freq)/number of ADCs) . So each one will sample the same input at another time point . It will better solution than previous , so here you mas sample even non periodical signal .