adding custom field to oData result

adding custom field to oData result

Calling an oData service from ajax I receive following object:
{"odata.metadata":"http://localhost:55878/odata/$metadata#Products","odata.count":"5",
"value":[
{
"ID":3,"Name":"Scarf","Price":"12","Category":"Apparel"
},
{
"ID":4,"Name":"Yo-yo","Price":"4.95","Category":"Toys"
}
],
"odata.nextLink":"[http://localhost:55878/odata/Products$inlinecount=allpages&$skip=4]"
}
notice I'm using pagging and that's why server return odata.nextLink url
in the odata object.
Now I want to add a custom field "odata.previousLink" in odata object.
where and how can I achive that ??