string rvtName = @"c:\work\file.rvt";
UIDocument uidoc = data.Application.OpenAndActivateDocument(rvtName);
RVT_DB.Document doc = uidoc.Application.ActiveUIDocument.Document;
RVT_DB.ElementFilter EF = CreateFamilySymbolFilter("К-1а", "К-1а");
var collector = new RVT_DB.FilteredElementCollector(doc);
var familySymbol = collector.WherePasses(EF).FirstElement();
RVT_DB.FamilySymbol fs = familySymbol as RVT_DB.FamilySymbol;
using (RVT_DB.Transaction tr = new RVT_DB.Transaction(doc, "add new elem sem"))
{
tr.Start();
doc.Create.NewFamilyInstance(new RVT_DB.XYZ(0, 0, 0), fs, doc.ActiveView);
tr.Commit();
}