As SRTM can be enhanced or might need some repairs a number of tools can be found on the web. Here some links are provided:

• Convert BIL to SRTM with BILxSRTM V2
• Convert them into SRTM with DEMxSRTM V1.3
• Use SRTMeditor to fill voids in SRTM3 with DTED1.