Just add more parameters to your querystring. You already have I1.Attributes.Add("src", "../MasterPage/Listpage.aspx?CompanyID=" + strCompanyid) so now just do
I1.Attributes.Add("src", "../MasterPage/Listpage.aspx?CompanyID=" + strCompanyid + "&parm1=" + Parm1)
You can then keep adding on more parameters as needed using the ¶mname=paramvalue format.