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I have a project consisting of several blocks in schematic interface, they are mainly custom blocks written in VHDL, what I want to do is, to include the Nios system in the schematic interface.
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So you have VHDL that you have connected together using a schematic?
Depending on how much work it would take, I would recommend modifying your VHDL so that each component has an Avalon-MM interface. Of course that depends on what your VHDL does, so if you can describe your existing components, I could recommend a solution.
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In SOPC, when I was trying to generate the Nios system, I will be prompted to select either Verilog or VHDL whenever I want to create a new Nios system, but I could not find this option in Qsys.
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Under the "Generate" tab or menu option (depending on Qsys version), you can select Verilog or VHDL for simulation, or on the newest version for synthesis. On older versions of the tools, synthesis only produced Verilog code. This was not really an issue, as even if you instantiated the Qsys system into a top-level VHDL file, all you had to do is add a component declaration.
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could I still combine the Nios system generated in Verilog to communicate well with all the custom blocks written in VHDl where I am going to combine all of them in schematic file?
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Yes, its no problem mixing Verilog, VHDL, and schematics. However, any serious developer should also be simulating their code in Modelsim. While it is possible to generate an HDL version of a schematic, I would recommend moving over to an entirely HDL development flow. Modelsim-SE can be used to simulate mixed Verilog and VHDL designs. If your NIOS II system is the only part of your design that is Verilog, then you can generate its simulation in VHDL, and have a 100% VHDL design that you can simulate in Modelsim-ASE.
Cheers,
Dave