What is anti-alias pre-filter for preventing aliasing after under-sampling?
I think you're looking for a free lunch that does not exist. Your original question and response to Peter K's answer suggests that you want to sample a signal that has both lowpass and highpass content, with the highpass content extending beyond the Nyquist frequency associated with your target sample rate. That is probably not going to work.

Given a sample rate $f_s$ (and real samples), you can only unambiguously represent frequencies on the interval $[0, \frac{f_s}{2})$. More generally, you can only represent a swath of bandwidth that is up to $\frac{f_s}{2}$ wide. Frequencies above the Nyquist rate alias down such that they appear to be located in this interval after you sample them. If you have a signal of interest that meets this bandwidth constraint, then you can use bandpass sampling techniques; basically you select a sample rate taking into account the center frequency and bandwidth of the desired signal. You allow the signal to alias in a "controlled" manner, so that it appears to be present in a contiguous portion of $[0, \frac{f_s}{2})$ after you sample it (perhaps with the spectrum inverted, but that is easily fixed).

This does not seem to line up well with what you seem to want. Your question indicated that you have lowpass content (i.e. content near zero frequency) that you want to preserve in addition to highpass content above $\frac{f_s}{2}$. In many cases, this is not going to be achievable without the highpass content aliasing down on top of the lowpass signal of interest after you sample. However, under certain conditions, you might be able to make this work. If:


The lowpass and highpass components are separated in frequency (i.e. there is a gap between the two regions where you don't care about preserving the signal's content),
You know the center frequency and bandwidth of the highpass portion (so it is more accurately termed "bandpass" instead),
And you have control over the sample rate,


Then you may be able to make it work. In that relatively special case, you would simply apply the bandpass sampling approach described before, except the sample rate must be selected with caution so that the higher-frequency content does not alias down into the portion of the band that the lowpass signal occupies.

Whether you would actually want to do this in a practical system is still an open problem. It's not clear specifically what you're trying to accomplish, or what the constraints in your application are. An alternate approach would be to separate the two signal components using analog filters (lowpass for one channel, highpass/bandpass for the other), then sample them independently. This could allow you to use a lower sample rate, commensurate with the bandwidths of each component.