Plasmoio is a plasmoid, written in PyKDE/QT that allow you to send SMS using shell command interface of well known program called MoioSMS (or its successor ermeSMS).
It uses accounts and contacts stored in config.ini file created by MoioSMS (or ermeSMS).
Plasmoio gives you the possility to add new contacts, storing them in right way inside config.ini file.
On the other side, it doesn't allow you to add new accounts.
So my suggestion is to use MoioSMS (or ermeSMS) to add them and restart Plasmoio to update new accounts.
Plasmoio is intentionally simple, just chose recipient and account, write text message and send it.
Only two usefull features: number of messages sended for each account during a day; completer in recipient box.
I tested it only with Vodafone sms and mms, and Tim sms accounts.
In general, if an account work in MoioSMS it will work also in Plasmoid.

Remember that to get available those account that need inserting text from some captcha, you must install gocr,ocrad and graphicmagick.

References:
http://www.moioli.net/sms/
https://github.com/sylarpowa/ermeSms