Yes you can Alfresco model is just and XML file ,you can manipulate the xml file usingthe dynamic deployment approach .That is placesing the xml files in the content repository under Company Home/Data Dictionary/Models.
Yes of course ,There is not much difference between operating nodes in Company Home/Data Dictionary/Models and operating other nodes.
You can just add nodes with xml content using webscript just as you add other nodes ,just make sure the xml content you upload is a legal model definition file.