Combining secret keys into a single shared key Let's say there are $n$ participants each with their own secret key $x_i$. Is it possible for them somehow to build a shared key which is an arithmetic sum of all their secret keys ($x_0 + x_1 + ... + x_n$) but without revealing their individual keys to each other.

Also, ideally, this would be done non-interactively. Meaning, each participant shares something about their secret key with everyone else, and once everyone has shared this "something", the shared key can be constructed.
Secret sharing is a perfect tool for this application. Under any linear secret sharing scheme, individuals can generate shares of secret values to distribute to other parties involved in the scheme. Once each individual has distributed shares of their secret value to all the other parties, due to the linearity of the scheme they can each locally sum the set of received shares and the collectively hold a valid share of the summation. This is because, by definition, linear secret sharing schemes possess and additive homomorphism, meaning that adding shares yields shares of a value which, if revealed in a reconstruction process, is identical to the value of the summed secrets. 

In a more detailed manner, illustrated with additive secret sharing, every party $P_i\ \forall i\in Z_{n+1}$ generates $n$ random values in some valid group denoted by $m$ which will serve as shares of the secret $x_i$ for all the other player i.e. $[x_i]_{\mathbb{Z_m}}^{P_j}\in_R\mathbb{Z}_m \ \forall i,j\in Z_{n} $ and the final share satisfies the following equation $[x_i]_{\mathbb{Z_m}}^{P_n}=x_i-\sum_{j=0}^{n-1}[x_i]_{\mathbb{Z_m}}^{P_j} \mod m$. Then every party $P_i$ sends $[x_i]_{\mathbb{Z}_m}^{P_j}$ to all other parties such that $i\neq j$. Finally, the parties all locally compute the following summation locally in which $x_\Sigma$ denotes the summed secrets. 
$$ [x_\Sigma]_{\mathbb{Z}_m}^{P_i} = \sum_{j=0}^{n}[x_j]_{\mathbb{Z}_m}^{P_i}\mod m$$  

Now each party holds uniformly random and information theoretically secure shares of the sum of their individual secrets and all parties must cooperate to reveal the underlying value. Specifically they would have to communicate all their individual shares to one another then they could each locally calculate:
$$x_\Sigma = \sum_{i=0}^{n}[x_\Sigma]_{\mathbb{Z}_m}^{P_i} \mod m$$

I encourage you to look deeper into secret sharing in general as well as other schemes, Shamir's secret sharing scheme in particular, due to the resilience it can introduce to a system in the presence of active adversaries or component failures.