I asked this exact same thing about a month ago. If there is anyway to just add ports to the default list on the fly.
Like nmap -p+4444,4445 x.x.x.x .. the short answer is no currently there is not. I did speak to one of the devs and they said throw a e-mail to nmap and they could probably put out a patch in a few days, but i never got around to it as i only needed it for a few scans.
Your best bet .. is to do a default scan, then do another scan with your custom ports and combine the output either by hand or witha custom script depending on what you want.
Alternatively you could do something like this ... grab all the default ports that nmap normaly uses and write it all out into a batch file, then just use that script to add in your ports like so
nmap -P0 -n -sV -T4 -p 21,22,23,-snip-,80,1531,$1 $2 -oA outty
Then just do ./nmap-script.sh 25000,250001 192.168.1.0/24