Is it possible to combine [FromRoute] and [FromBody] in ASP.NET Core?

It’s doable by a custom model binder as mentioned in the comment. Here is a few code snippets to wire everything up, with the example you can send a http request with the following JSON body to an API /api/cats?From=james&Days=20

{
    "Name":"",
    "EyeColor":"Red"
}

A few classes, you can find them here as well: https://github.com/atwayne/so-51316269

// We read Cat from request body
public class Cat
{
    public string Name { get; set; }
    public string EyeColor { get; set; }
}

// AdoptionRequest from Query String or Route
public class AdoptionRequest
{
    public string From { get; set; }
    public string Days { get; set; }
}

// One class to merge them together
[ModelBinder(BinderType = typeof(CatAdoptionEntityBinder))]
public class CatAdoptionRequest
{
    public Cat Cat { get; set; }
    public AdoptionRequest AdoptionRequest { get; set; }
}


public class CatAdoptionEntityBinder : IModelBinder
{
    public async Task BindModelAsync(ModelBindingContext bindingContext)
    {
        // Read Cat from Body
        var memoryStream = new MemoryStream();
        var body = bindingContext.HttpContext.Request.Body;
        var reader = new StreamReader(body, Encoding.UTF8);
        var text = reader.ReadToEnd();
        var cat = JsonConvert.DeserializeObject<Cat>(text);

        // Read Adoption Request from query or route
        var adoptionRequest = new AdoptionRequest();
        var properties = typeof(AdoptionRequest).GetProperties();
        foreach (var property in properties)
        {
            var valueProvider = bindingContext.ValueProvider.GetValue(property.Name);
            if (valueProvider != null)
            {
                property.SetValue(adoptionRequest, valueProvider.FirstValue);
            }
        }

        // Merge
        var model = new CatAdoptionRequest()
        {
            Cat = cat,
            AdoptionRequest = adoptionRequest
        };

        bindingContext.Result = ModelBindingResult.Success(model);
        return;
    }
}


// Controller
[HttpPost()]
public bool Post([CustomizeValidator]CatAdoptionRequest adoptionRequest)
{
    return ModelState.IsValid;
}

public class CatAdoptionRequestValidator : AbstractValidator<CatAdoptionRequest>
{
    public CatAdoptionRequestValidator()
    {
        RuleFor(profile => profile.Cat).NotNull();
        RuleFor(profile => profile.AdoptionRequest).NotNull();
        RuleFor(profile => profile.Cat.Name).NotEmpty();
    }
}

// and in our Startup.cs
public void ConfigureServices(IServiceCollection services)
{
    services.AddMvc().AddFluentValidation();
    services.AddTransient<IValidator<CatAdoptionRequest>, CatAdoptionRequestValidator>();
}