Further to Martin's comments, I guess it would depend on which type of CPAP set up you are using. Some CPAP units have attachments that allow MDIs to be inserted/used as required. Alternatively, as Martin suggests there is the possibility of inserting traditional nebuliser units into the CPAP circuitry, although flow rates, particle size, deposition etc are all issues to contemplate. At the ICU/ high dependency end of the spectrum there are numerous new generation ventilators that allow 'smart' nebulisation (on inspiration only to avoid wastage) during CPAP (whether delivered invasive or non-invasive) - these can be set up via infusion pumps to allow almost continuous nebs to be delivered, which can be very effective and useful in the very acute asthmatic.
Perhaps hasanj you could describe the equipment (manufacturer, model etc) to help guide our thoughts and input if we have experiences with similar equipment?
Happy New Year! (well almost!!)